Biosensors that can quantify a specific object to be measured in a sample liquid in a simple and rapid manner, for example, without diluting or stirring the sample liquid have been used widely. Such a biosensor can be produced by, for example, forming an electrode system having a working electrode (also referred to as “measuring electrode”) and a counter electrode on an electrically insulating substrate by a method such as screen printing, and forming a reactive layer including a redox (oxidation-reduction) enzyme and an electron-accepting substance that react with the objective to be measured thereon. When the reactive layer is in contact with the sample liquid containing the object to be measured, the object to be measured is oxidized by the catalytic action of the redox enzyme, and the electron-accepting substance is reduced at the same time. The reduced electron-accepting substance is re-oxidized by an electrochemical approach, and the concentration of the object to be measured in the sample liquid can be calculated from the thus obtained oxidation current values.
However, there may be errors in the measurement, depending on the properties of the sample liquid or the like. Possible reasons for this are as follows. For example, a whole blood sample contains impurities, for example, solid matter such as blood cells, soluble components such as lipid, protein, and saccharides, and insoluble components. The area of the surface of the electrodes may be decreased by the adsorption of these impurities onto the surface of the electrodes, or the impurities may hinder the diffusion of the reagent and inhibit the enzyme reaction. Consequently, the current value is decreased. Furthermore, there is a large difference between individuals in the hematocrit (Hct) value, which is a volume ratio of erythrocytes to whole blood, so that there is a difference between specimens in the above-described influence on the biosensor. Such an influence due to the impurities can be decreased by, for example, diluting the sample liquid and then subjecting the diluted sample liquid to a biosensor. However, this takes more time and complicates the operation.
In order to avoid such an influence, the following biosensors have been proposed: a biosensor (JP9-80010 A) in which an immobilized enzyme film containing mutually charge-coupled redox enzymes and chitosan is disposed on a detection surface of the electrode system; and a biosensor (IP10-113200 A) in which a layer comprising microparticles of a water-soluble high molecular weight compound and conductive particulates is disposed. The microparticles contain an enzyme. These biosensors attempt to decrease the influence due to the impurities by providing the above-described various layers as a filtering film, and bringing the sample liquid into contact with the electrode surface via the filtering film to suppress the impurities in the sample liquid from coming close to the electrode surface.
Such a biosensor can filtrate impurities such as erythrocytes or proteins that hinder measurement. However, since the filtering film is provided, the permeation of the sample liquid is not uniform, so that the electrode surface may not become sufficiently wet. Therefore, for example, air bubbles remain on the electrodes, so that an effective measurement area of the electrodes is decreased, which may cause measurement errors. Furthermore, the permeation of the sample liquid takes time, and therefore the response speed is slow.